With decades of technical advancements and breakthroughs in the areas of semiconductor based solid state light emitting devices, Edison's incandescent light bulbs, which typically have 30% or less light efficiency, will soon be replaced with energy-efficient light emitting diodes (“LEDs”). A conventional LED is small and power efficient with good lifetime. Various commercial applications of LEDs, such as traffic lights as well as electronic billboards, have already placed in service.
An LED is a semiconductor diode with a biased p-n junction capable of emitting narrow-spectrum light or electroluminescence. Color of emitted light typically depends on the composition of material used in the device. Color variations for visible light are usually defined by electromagnetic radiation or optical wavelengths from approximately 400 nm (nanometer) to 700 nm. The quality of a light source on color appearance of objects is usually measured by the Color Rendering Index (“CRI”), wherein the highest CRI rating is 100. Typical cool white fluorescent lamps, for example, have a CRI of 62. On the other hand, lamps having rare-earth phosphors can be constructed with CRI of 80 or better.
A problem associated with a typical LED for indoor lighting is a low CRI rating, which could distort true color of objects. To enhance the color appearance, a conventional approach is to mix red phosphor with yellow phosphor to generate warm or neutral white light. Another conventional approach is to dispense two continuous red and yellow phosphor layers one on top of another to generate warm or neutral white light. A drawback, however, associated with these approaches is low light output (“LOP”) or low lighting efficiency. LOP can be defined as a measurement of light luminosity, luminous flux, luminous intensity, light brightness, and the like.
Another approach to improve CRI rating is to combine yellow LEDs capable of emitting cool light with red LEDs capable of emitting red light. The combination of cool white light with red light enhances CRI rating. A drawback, however, associated with this approach is that the process of combining red LEDs with yellow LEDs involves a complicated assembling procedure. In addition, combining red and yellow LEDs may require additional circuitry on a control circuit board.